1. Field of the Invention
The present invention relates to a power-supply unit, and more particularly, to a power-supply unit which suppresses harmonic current and improves power-factor.
2. Description of the Related Art
In equipment receiving the supply of electric power from a commercial alternating current power supply, the power supply is required to suppress the generation of harmonic currents. Since a circuit having a harmonic current suppression function also includes a power-factor improvement function, the circuit is often called a power-factor improvement circuit as in a power-supply unit shown in Japanese Unexamined Patent Application Publication No. 2003-18842 (Patent Document 1).
In the equipment having a standby mode as in televisions, for example, it is also necessary to operate a waiting circuit (for example, a reception circuit for receiving only a power-supply upon receipt of a signal from a remote control system) during standby. However, it is very inefficient to operate the entire power-supply unit for a waiting circuit which consumes only a very small amount of electric power. In particular, since a reduction in electric power loss during standby has recently been required, it is necessary to develop countermeasures.
In a switching power-supply circuit as described in Patent Document 1, a main power-supply circuit which is not operated during standby and a secondary power-supply circuit which is operated to constantly supply a small amount of electric power, including during standby also, are provided, and both the main power-supply circuit and the secondary power-supply circuit are connected to an alternating current power supply.
In the switching power-supply circuit described in Patent Document 1, since large harmonic current is generated in the main power-supply circuit having a large electric power supply capacity, even if the circuit structure becomes complicated, a harmonic current suppression circuit is provided therein, and, on the contrary, since small harmonic current is generated in the secondary power-supply circuit having a small electric power supply capacity, no harmonic current suppression circuit is provided therein.
In equipment, such as televisions, there are cases in which a power supply having various output voltages is required, and there is the demand for a secondary power-supply circuit which can be used not only on standby, but also for other purposes during normal operation. Practically, necessary voltages on standby are often the voltages of +5 V and +3.3 V required for operation of simple digital circuits, for example, the voltages of +5 V and +3.3 V can also be used for various other purposes. Therefore, a power-supply unit is designed so as to be fitted not for the amount of power consumption on standby, but for the amount of power consumption during normal operation. In this case, the secondary power-supply circuit is designed so as to have a substantial amount power supply capacity in reserve.
However, in Patent Document 1, for example, the secondary power-supply circuit includes no harmonic current suppression circuit. Accordingly, when the amount of power supply in the secondary power-supply circuit increases, the amount of harmonic current generated also increases, and this increased harmonic current is not accounted for.